This invention relates to an apparatus and method for forming composites, and particularly to forming layers of certain types of carbide coatings.
Layers of carbide materials such as titanium carbide (TiC), boron carbide (B.sub.4 C) and silicon carbide (SiC) are often coated on surfaces for various commercial and scientific applications. Such surface coatings can be used in the tool making industry to produce super hard coatings on alloy tools. Such composites find particular utility in producing cutting tools and in providing wear resistant coatings for parts. Various approaches toward forming such coatings are presently known. However, the prior art approaches have limitations in terms of the surface quality of the coating which can be developed, and in the deposition rate of forming the coatings.
One previously known approach of providing a titanium carbide coating is through plasma spraying. In plasma spraying, titanium carbide powder is introduced into a plasma jet where it is intensely heated to melting and sprayed to form a coating on the part surface. The thickness of coatings produced by plasma spraying is from tens of microns up to several millimeters. However, the materials produced by this method tend to have a high porosity; more than 10%. For this reason it is recommended to use dispersed powders to reduce the coating porosity, but it is difficult to introduce such agents into the plasma jet.
Also know is a process of developing a titanium carbide layer through detonation coating. This process, however tends to produce low quality coatings.
Another known process is the use of a direct electron beam evaporation of titanium carbide. Billets for evaporation are produced by a powder metallurgy process. The billets are evaporated from a water-cooled crucible. The maximum titanium carbide deposition rate which can be achieved by this method, however, is on the order of 0.3 .mu.m/min, and thus this process is not promising for industrial applications where relatively thick coatings are often required.
Activated reactive evaporation is another known technique for producing a titanium carbide coating. Evaporation is caused by an electron beam. At the same time, a reaction gas such as methane is fed into a chamber at a low pressure. Titanium carbide is formed both in the gas phase, and on the substrate as a result of the reaction between the metal vapor and gas atoms: EQU 2Ti+C.sub.2 H.sub.2 .fwdarw.2TiC+H.sub.2
A charged grid is placed in the working space of the chamber to activate the chemical compound formation. By changing the reagent partial pressure it is possible to change the carbon-titanium ratio. The rate of titanium carbide coating deposition generally does not exceed 2.5 .mu.m/min using this method.
Still another method for developing carbide coatings is based on electron beam evaporation of the initial component, e.g. titanium and carbon from separate sources and performing carbide synthesis on the substrate. The ratio of components in the coating can be controlled by changing the ratio of heating power of the materials evaporated from the sources. Low rates of initial component evaporation are the restrictive factor in the utilization of this electron beam process for titanium carbide-based coating deposition. Problems with this method are encountered when attempts are made to raise the deposition rate. Raising the power of heating of the carbon source above a critical value results in a marked increase in the number of graphite fragments in the vapor phase, and, as a result, in the appearance of defects, discontinuities and availability of free carbon in the condensate; resulting in lower quality of the resulting coating. Similarly, when attempts are made to increase the titanium evaporation rate by raising the heating power, intense spattering of the molten metal occurs which again degrades the ultimate surface quality.
Advances have been made in terms of enabling the evaporation rate of carbon heated by an electron beam to be increased without encountering solid carbon particles in the vapor. A molten intermediate pool of tungsten can be employed overlying the carbon source which prevents the direct heating of the graphite surface by the electron beam and provides the dissolution of the graphite particles through the formation of a tungsten carbide melt. Since the vapor pressure of carbon is higher than that of tungsten at a given temperature, it is primarily the carbon that evaporates from the melt. This advance, however, does not translate into a higher rate of titanium carbide deposition since the limiting factor remains the rate of titanium evaporation. Consequently, without raising the titanium evaporation rate the coatings produced by this method providing excess carbon results in a coating of titanium carbine and elemental carbon.